A 15-centimeter terrestrial Entomopter 

Context in source publication

Context 1

... to carry out useful missions has various advantages including: N extended range because high frequency line-of- sight links are obviated, N quicker reaction time to atmospheric perturba- tions and obstacle avoidance than can be afforded by a teleoperator, N potentially greater stealth due to lower bandwidth emissions, N the ability to operate indoors or in urban canyons where communication is not possible, N jam resistance, N the potential for beneficial emergent behaviors leading to higher probability of mission success. Difficulties in testing these tiny MAVs fall into two categories when they suddenly are given the power of autonomy: physical flight testing and behavioral testing. To exemplify some of the issues involved in each, consider the flapping wing MAV known as the Entomopter. The Entomopter was designed from inception to be a fully autonomous MAV for use in indoor reconnais- sance. Initial development began at the Georgia Tech Research Institute by the author under an independent research and development program and was later funded by the Defense Advanced Research Projects Agency’s Mesomachines program to demonstrate feasibility of such a device for indoor flight. The U.S. Air Force Research Laboratory then provided funding to extend the Entomopter’s chemically-fueled propulsion system into its fourth generation. Subsequently, the National Aeronautics and Space Administration (NASA) Institute for Advanced Concepts became interested in the Entomopter’s unique flight capabil- ities, which make slow flight in the lower Mars atmosphere possible. A feasibility study was then funded to show how an Entomopter-based Mars Surveyor could enhance the science missions envi- sioned for Mars. All of these programs involved analytical substantia- tion of the Entomopter in various environments ranging from low Reynolds number flight in the lower atmosphere of Earth to low Reynolds number flight in the lower atmosphere of Mars. Also common to these programs was the fact that the Entomopter was to be fully autonomous and never teleoperated. Full autonomy was essential for indoor operation where communication and global positioning system signals were not available, and it was likewise essential for Mars operations where the latency of control (10–15 minutes depending upon the distance of Mars from Earth) necessitated a vehicle that could carry out missions unassisted. The terrestrial Entomopter (Figure 1) is a multi- mode autonomous robot capable of flight and limited ambulatory behaviors. Autonomous navigation is based on a combination of attraction and avoidance behaviors deriving input from both an integrated optic-olfactory sensor for detection of chemical species (or, alterna- tively, a sensor for a specific type of radiation), and an ultrasonic swept beam ranging device. The terrestrial Entomopter eventually found potential applications on Mars by virtue of its unique ‘‘blown’’ flapping wing (U.S. Patent No. 6,082,671 and U.S. Patent No. 6,446,909). Present planetary surface rovers have shortcomings that NASA could address with a slow flying aerial platform, however flight on Mars is complicated by the fact that the atmosphere is rarefied, thereby making it difficult to generate lift with conventional wings. In fact, fixed wing vehicles must have enormous wings and travel at speeds in excess of 300 kilometers per hour to stay aloft in the Mars atmosphere (Colozza et al. 2000). Turn radii are on the order of kilometers, making it inefficient to return to points of interest, and high-speed traverse across the surface at lower altitudes causes smearing of sensor data, thereby negating any beneficial increase in resolution that may have otherwise been gained (Colozza et al. 2002). NASA recognized that the ability of the Entomopter to fly in low Reynolds number conditions without the need for air-breathing propulsion made it a natural candidate for flight in the rarefied atmosphere of Mars, albeit in a larger incarnation. Unlike fixed wing flyers, an Entomopter-based Mars surveyor would be able to cover a wide area while still being able to fly slowly and return to a refueling rover. The Entomopter began as a biologically inspired design, but rather than attempting to replicate biological kinematics and aerodynamics, improved systems have been devised to leverage what is observed in biological systems to produce a machine that is manufacturable, controllable, and able to generate the power necessary to fly from onboard energy sources (Michelson 2004). The Hawk Moth (Manduca sexta ) was chosen as a baseline model for the wing aerodynamics. The University of Cambridge in England was part of the initial Entomopter design team because it had studied Hawk Moth wing aerodynamics for more than a quarter of a century and had produced seminal works describing the leading edge vortex and its effects on the flapping wing (Ellington et al. 1996, Liu et al. 1998, van den Berg and Ellington 1997, Willmott and Ellington 1997, Wilmott et al. 1997). The flapping mechanism for the Entomopter has been extended beyond that of the Hawk Moth to provide a resonant single-piece construction that takes advantage of torsional resonance in the Entomopter fuselage to recover flapping energy common to flying insects that temporarily store potential energy in either muscles or exoskeletal parts (resilin). In the terrestrial version, the same structure that provides wing flapping also scans a frequency modu- lated continuous wave ultrasonic beam to provide front, side, and down-looking range measurements for obstacle avoidance and altimetry. It also has the potential to track and follow free-moving agents in a fashion similar to that employed by bats. Stability and control in flight as well as navigation are achieved by actively modifying the lift of each wing on a beat-to-beat basis using pneumatic control of the air circulating over the beating wing. Also, as demonstrated in the Georgia Tech Research Institute’s wind tunnels where pneumatically controlled wings were shown to develop positive lift at negative angles of attack ( a ) as great as 2 70 degrees (Englar et al. 1994), Entomopter wings (unlike those of the Hawk Moth) should be able to generate positive lift not only on the downbeat but the upbeat as well. These wind tunnel tests have shown that coefficients of lift exceeding the theoretical maximum by 500 percent for the given wing shape can be achieved without the complexity of active angle-of-attack modulating mechanisms (Michelson and Naqvi 2003). A chemically fueled reciprocating chemical muscle has been designed and is in its fourth generation of development. This actuator system has demonstrated 70-Hz reciprocation rates with throws and evolved power levels necessary to support flight of a fully autonomous Entomopter system (Michelson and Amarena 2001). The reciprocating chemical muscle uses the energy locked in various monopropellants to produce reciprocating motion for propulsion as well as waste gas products for the operation of gas bearings, an ultrasonic obstacle avoidance ranging system, and full flight control of the vehicle. Rigorous testing of MAVs usually begins in a wind tunnel with airfoil sections and eventually with the entire air vehicle. A problem in wind tunnel testing is that many tunnels are not configured to handle such small test objects and the balances may not have the desired ...